docs: add FEAT-04 4G LTE modem (Huawei E3372h-153) to replace SIM868 2G hat

FEATURES.md

@@ -60,6 +60,34 @@ dedicated PLAN.md phase (or fold it into the relevant existing work) and mark it
   for a live OBD reading; decide whether it should also sync to the remote device config
   (Phase 25.5) or stay local.
 
+### FEAT-04 — Replace SIM868 2G GSM hat with 4G LTE modem (Huawei E3372h-153)
+- **Status**: `IDEA`
+- **Area**: modems / uplink (Pi `modules/app_cellular.py`, `modules/app_gps.py`, uplink
+  switching) — replaces the current SIM868 GSM/GNSS HAT + cellular fallback path
+- **Phase**: n/a
+- **Requested**: owner — just purchased a Huawei **E3372h-153 4G LTE Modem**. The 2G/3G-only
+  SIM868 hat will not work in Canada (2G networks are decommissioned), so the plan is to make the
+  4G modem the working cellular uplink instead.
+- **Idea**: integrate the Huawei E3372h-153 as the primary cellular data uplink, replacing reliance
+  on the SIM868's GSM (AT+HTTP) path. The E3372h-153 is a USB "stick" modem — likely supported via
+  Realtek / cdc_ether / usb-storage based drivers and exposes a dial-up or LTE dongle interface the
+  Pi can use for connectivity (e.g. via NetworkManager / ModemManager, or a static APN config).
+- **Outstanding considerations**:
+  - Determine the E3372h-153's local mode vs stick/dial mode and the correct Linux driver/interface
+    (hierarchical USB mode switching may be needed; it's the "153" variant which may use the RNDIS /
+    ECM / CDC-NCM profile).
+  - Whether to run it as the *primary* uplink or just another option in the existing phone-hotspot /
+    home-wifi / local-AP priority chain.
+  - What happens to the SIM868 — keep it purely for its **GPS/GNSS** role (it's the PWRKEY-toggled
+    GNSS receiver on `/dev/serial0`) while dropping its **2G GSM** cellular path, vs removing it
+    entirely.
+  - The SIM868's `app_gps.py` GNSS positioning must keep working (it's the car's GPS source) even if
+    the 2G-modem half is retired — separate the two concerns.
+  - **Keep this feature as-is (2G/GSM fallback) for countries where 2G/3G still works** — don't
+    remove the existing cellular fallback code; gate it so the 2G path remains available there.
+  - LTE SIM plan/APN specifics for Canadian carriers (Rogers/Bell/Telus) and whether the modem can
+    run the Pi's hotspot/SSID the same way `app_network.py` currently does.
+
 ---
 
 ## Ideas that landed